The sexually transmitted disease, gonorrhea, poses a worldwide risk as one of the most commonly reported communicable diseases. Gonorrhea is caused by the bacterium Neisseria gonorrhoeae, a gram negative diplococcus. Although the pathogen primarily infects mucous membranes, it is capable of invading tissues and evading host defenses. N. gonorrhoeae is the causative agent of a spectrum of sequelae. These range from asymptomatic mucosal infection to significant disease syndromes in both men and women. The more serious of such syndromes include, for example, disseminated gonococcal infection (“DGI”) in men and women, as well as salpingitis or pelvic inflammatory disease (“PID”) in women. Either salpingitis or PID may themselves lead to long-term sequelae, including ectopic pregnancy and infertility. Other important sequelae, sometimes requiring surgical intervention, include recurrent infection, chronic pelvic pain, dyspareunia, pelvic adhesions and other inflammatory residua.
It has been estimated that in the United States, the direct and indirect costs of treating PID and associated ectopic pregnancy and infertility totaled 2.6 billion dollars in 1984 (53). The total direct costs were estimated to be 2.18 billion dollars in 1990, with indirect costs of 1.54 billion dollars. Assuming constant inflation and incidence rates of PID, the total cost of this disease is projected to reach 8 billion dollars in the year 2000 (9).
Despite public health efforts to control gonococcal infections and the availability of effective antibiotic therapies in the United States, there are approximately 315,000 cases of gonorrhea reported annually to the Centers for Disease Control (“CDC”) (12). A substantial proportion of all cases of gonorrhea occur in asymptomatically infected individuals who are the source of most new cases within a community (6). The increasing prevalence of antibiotic-resistant strains has complicated treatment of the infection (10, 11, 52).
N. gonorrhoeae has multiple virulence factors. The surface components of this pathogen play an important role in attaching to and invading host cells, while providing potential targets for the host immune response. Gonococcal infections elicit local and systemic humoral and cellular immune responses to several components which are exhibited as surface exposed antigens of the bacterium, particularly pili, porin (“Por”) or protein I (“PI”), opacity associated proteins (“Opas”) or protein IIs, Rmp or protein III, and lipooligosaccharides (“LOSs”) (7). Pili, Opa, Por and LOS are all implicated in attachment to and invasion of the host and all display considerable variation on their surface exposed regions (26, 45, 46). The intra- and inter-strain variations of gonococcal surface components have led to hypotheses regarding tissue specificity at different sites and the organism's potential for reinfection and continued virulence.
In both symptomatic and asymptomatic patients, gonococcal infections have been shown to stimulate increased levels of anti-gonococcal serum immunoglobulins. The peripheral humoral response is predominately IgG (mostly subclass IgG3), with lesser amounts of IgM and IgA (13). Quantitatively, the antibody response is primarily directed against the pili, Opa proteins and LOS. Local antibodies are present in genital secretions, but in reduced amounts (48), and may be directed against different antigenic targets than those in serum (27). The predominant class of antibodies present in secretions is also IgG (mostly IgG3) and not secretory IgA (“sIgA”) (7). Antibodies against LOS are present as well, but in lesser amounts than those against pili, Por and Opa. Although patients infected with N. gonorrhoeae may show an antibody response to many gonococcal antigens, N. gonorrhoeae isolated from patients with disseminated infection (DGI) are resistant to the bactericidal action of normal human serum (“NHS”) and of most convalescent sera (38). This serum-resistant phenotype, termed stable serum resistance (“SR”), may enable the organism to evade local defenses, penetrate mucosal barriers and disseminate via the bloodstream.
Upon subculture, many strains of gonococci become phenotypically sensitive to killing by NHS or serum sensitive (38). These organisms are termed serum sensitive (“SS”) or unstably serum-resistant. Such organisms are frequently isolated from women with severe manifestations of local inflammation or clinically evident PID. Acute salpingitis, the pathologic counterpart of PID (caused by SS gonococci), rarely progresses to bacteremic illness or DGI. This suggests that the intense local inflammatory response, generated by SS gonococci, may serve to contain the infection and prevent bacteremia, although at the cost of damaging the local tissues. SS gonococci generate significantly greater amounts of the complement derived chemotactic peptide, C5a, than do SR gonococci (16). This may be responsible for the polymorphonuclear leukocyte (“PMN”) mediated inflammatory response that is produced by SS gonococci.
The development of antibiotic-resistant strains of N. gonorrhoeae, has rendered control of this infection increasingly difficult. The potential to undertreat gonococcal infection has accelerated the need for an anti-gonococcal vaccine. The prevention of gonococcal infection, particularly the severe complications of PID, has been the goal of many investigators. Ongoing attempts to develop an effective anti-gonococcal vaccine, however, have been plagued with several difficulties.
Attempts to use individual surface components of the pathogen as targets for conventional vaccines have been unsuccessful because of their antigenic variability. Pilus vaccines have been protective only against infection with the homologous strain (used to make the pilus vaccine) and Por vaccination has been unsuccessful even in human experimental challenge. In addition, N. gonorrhoeae express marked phenotypic heterogeneity, typically shifting from one antigenic form to another at a frequency of >1 in 103 organisms (49, 50) making the surface of this organism a moving target for most vaccine strategies. Although the vaccine candidates have provoked antibody responses, the antibodies and immune responses produced have not been broadly protective.
LOS is an important virulence determinant of N. gonorrhoeae. Considerable evidence supports the role of LOS as a major target of bactericidal antibody directed to the surface of N. gonorrhoeae (2, 16, 18, 37, 47). Antibodies to LOS have several important functions: bactericidal activity, complement activation through the classical or alternative complement pathways (2), and opsonic activity (16). Additionally, LOS has been shown to be the most effective gonococcal antigen to induce a functional antibody response to homologous and heterologous gonococci (51).
The monoclonal antibody (“mAb”) 2C7 (30), detects a LOS derived oligosaccharide (“OS”) epitope that appears to be widely conserved and expressed amongst clinical isolates of gonococci. Typically, saccharides are T-cell independent antigens. When administered alone as immunogens, they generally elicit only a primary antibody response. In addition, oligosaccharides are small (<10 saccharide units) (19), and would likely require additional biochemical derivatization to render them immunogenic. The use of such oligosaccharides as vaccine candidates, therefore, is limited in several respects.
Internal image determinants have been proposed for use in vaccines (36). By means of mAb technology, a protective antibody (Ab1) to an epitope of interest on the pathogen can be produced. The particular antibody (Ab1) can be purified and subsequently used as an immunogen to elicit an anti-idiotypic antibody (Ab2) which may be an internal image of the original epitope on the pathogen.
As predicted by the Jerne “network” theory (23), immunization with an anti-idiotypic antibody (Ab2) that is directed against antigen combining sites of primary antibody (Ab1), may elicit a humoral immune response specific for the nominal antigen. The resulting anti-anti-idiotypic antibody (or Ab3) should react with the original primary antigen. If the primary antigen is an oligosaccharide (and therefore expected to give a T-cell independent immune response), then immunization with Ab2 (the protein equivalent) may elicit a T-cell dependent response.
It has been demonstrated that an anti-idiotope of mAb 2C7 elicits anti-LOS antibodies in mice and rabbits that together with complement are bactericidal for gonococci, and that serum from animals immunized with this anti-idiotypic antibody also supports opsonophagocytosis by human PMNs (20).
It has also been shown that synthetic peptides which mimic a nominal antigen through binding to a specific antibody directed to the nominal antigen can also elicit an immune response against the nominal antigen (29, 24, 54).
The need exists for an agent useful for the prevention of gonorrhea targeted to the prevention of gonococcal salpingitis, an infection that may be associated with debilitating and chronic pelvic pain, infertility and ectopic pregnancy (42). Another important objective is to prevent transmission of the organism from an infected but asymptomatic host to an otherwise immune sexual consort. This is important because a substantial fraction of all cases of gonorrhea in both men and women are asymptomatic, and asymptomatically infected, sexually active persons are probably the major source of most new infections. Accordingly, a gonococcal vaccine that only attenuates the severity of symptomatic gonorrhea could result in a higher ratio of asymptomatic/symptomatic cases and as a result, such a vaccine might promote the spread of gonorrhea, unless it also prevents transmission (41).